1. Field of the Invention
The present invention is related to the field of formation logging, more particularly, to the verification and calibration of helium-3 (.sup.3 He) proportional counter detectors utilized in nuclear logging.
2. State of the Related Art
Nuclear logging techniques are well known and widely utilized by oilfield service companies to provide information necessary for oil and gas exploration and production decisions. Wireline nuclear logging techniques typically call for a logging sonde, having a neutron source and at least one neutron detector, to be lowered on an armored multi-conductor cable into a well borehole. The neutron source provides a source of high energy neutrons and may consist of an isotopic source, such as americium.sup.241 -beryllium, or neutron accelerator source. The formation of interest is irradiated by the high energy neutrons, which undergo collisions with the nuclei of naturally occurring formation materials. As a result of these collisions, the high energy neutrons lose some of their energy. The amount of energy lost by the high energy neutrons is inversely proportional to the size of the nucleus of the atom with which it collides. The collisions are generally elastic. Collisions with heavy nuclei result in relatively little energy loss; whereas collisions with a nuclei of approximately the same atomic weight, primarily hydrogen nuclei, result in a greater energy release. The high energy neutrons continue to lose energy until they slow to thermal velocities, which at room temperature, have an average energy of 0.025 electron volts (eV). This decay process occurs over a few microseconds. Once the emitted neutrons have slowed to thermal energy levels, they diffuse in the formation and are eventually captured by the nuclei of other elements in the formation. This capture of the thermal neutrons results in the emission of high energy capture gamma rays by the formation atoms.
The type of detector used within the logging sonde is dependent on the data of interest. Where thermal neutrons are to be counted for a porosity determination, for instance, the detector may be a helium-3 proportional counter type. The theory and operation of .sup.3 He proportional counters is well known and is exemplified in U.S. Pat. Nos. 3,240,971 or 3,102,198; or in the article "Recent Improvements in Helium-3 Solid State Neutron Spectrometry" authored by T. Jeter and M. Kennison, IEEE Transactions on Nuclear Science, February 1967, vol. NS-14 No. 1, pp. 422-27 ("Jeter"); or the book "Radiation Detection and Measurement" by G. Knoll, pp. 533-34 (1979) ("Knoll"). Other types of counters, such as sodium-iodide scintillation detectors, may be utilized to detect not only high energy neutron decay but capture gamma ray emissions as well.
Helium-3 detectors are commonly used to detect thermal neutrons. However, their use raises a number of issues. First, the detector response within the tool must be calibrated, i.e., the detector response must be properly characterized such that the detected thermal neutron distribution is properly centered about the known thermal neutron distribution peak of 0.765 MeV. This process is sometimes referred to as gain stabilization. Prior methods called for calibration of the detector in the field. This necessitated exposing the detector to a source of high energy neutrons, typically an isotopic source external to the sonde, to test detector response. This field procedure created a number of safety hazards. A field crew could be required to transport not only the source for the sonde, but a separate source strictly for the purpose of calibrating the sonde. In some instances, the sonde source was itself utilized for verification. It should be noted that safety concerns generally preclude the activation of a neutron accelerator at the surface for check source procedures, thereby necessitating a separate check source. As logging sonde design improved, it was no longer necessary to calibrate detectors at the well site since it could be performed at the logging company maintenance facility where specialized equipment could be utilized to reduce safety hazards.
Second, it is necessary to verify detector operation following transport to the well site. It will be appreciated that the transport of this specialized equipment to the job site could damage the detector or its related electronics. Because a .sup.3 He detector does not generally respond in the absence of a radiation source, a radiation source is required at the job site to verify that the detector is operating. Further, it is necessary to verify that the detector is responding to a source having a known energy distribution, characteristic peak and activity or count rate. It will be appreciated that the detector could fail, by way of open circuit or power failure, and that noise could be induced in the detector circuitry which could be mistaken for valid counts. Verification is achieved by measuring detector response to a source having a known energy peak and activity or count rate. The count rate is a function of the radioactivity level and may be readily determined by selection of the amount of source material.
One method of addressing this problem is disclosed in U.S. Pat. No. 5,180,197 to Wraight. Wraight teaches a self-calibrating/self-verifying .sup.3 He detector for use in the wireline and logging while drilling environments. Wraight teaches a .sup.3 He detector utilizing superatmospheric .sup.3 He gas having less than one part in 10.sup.10 tritium and, preferably, less than one part in 10.sup.11 tritium. Helium-3 detectors require this superpure .sup.3 He gas because the build up of beta particle emissions from tritium could conceivably create a noise which would become indistinguishable from the desired neutron counts. Wraight's .sup.3 He detector includes a self-calibrating feature which calls for the introduction of a low intensity source of alpha particles, such as a uranium or americium foil having a radioactivity level on the order of 10-30 nanocuries, in the body of the .sup.3 He detector. Most alpha particle energies occur in the range of 4 to 6 MeV, with a distribution peak of approximately 4.4 MeV, at the high end of the range of the desired thermal neutron distribution. Because the activity level of the alpha source can be readily controlled by proper selection of the amount of alpha source material, the low level alpha source provides a check source at a known energy and activity level well above the background noise which might be picked up by the detector. Further, because the low level alpha source is encased in the .sup.3 He detector, which itself is shielded by the sonde body, it is not necessary to utilize bulky source transport equipment when verifying the detector operation at the job site.
However, there still exist some problems with the self-calibrating .sup.3 He detectors disclosed by Wraight. First, the suggested alpha sources, uranium and americium, remain on the Nuclear Regulatory Commission (NRC) non-exempt list, i.e., there is no minimum amount or radioactivity level for these isotopes which is exempt from safety licensing standards. See, 10 C.F.R. .sctn..sctn.30.11-.20, 30.70 Schedules A and B. Should the sonde become stuck and unretrievable in a borehole, it would be necessary to take remedial safety measures to seal the sonde, even if the primary isotopic neutron source is retrieved from the sonde.
Second, counts attributable to the alpha source could affect the observed distribution of thermal neutrons. While efforts may be made to correct any distribution skew arising from the alpha source, it would still be difficult to determine at what level the thermal neutron count on the high end of the energy spectrum is being affected by the alpha source.
Third, the detector response to the alpha source must be determined and subtracted from the total detector counting rate to determine the count rate attributable to thermal neutrons. The counts from the alpha source introduce a statistical uncertainty in the thermal neutron count rate when the two count rates are similar.
Thus, there remains a need for a self-verifying, self-calibrating .sup.3 He detector which does not utilize a non-exempt radiation source and does not adversely affect the distribution of the thermal neutron count.